When uniform cylindrical pipe is employed in very deep waters, especially beyond about 3,000 feet depth, the pipewall becomes very thick and the pipe becomes quite heavy. At these depths, pipewall thickness is governed by external collapse pressure, which implies that the wall thickness increases rapidly with depth, even for relatively high grades of steel. Cost for such pipe is very high, welding of field joints is tedious and expensive, and handling of heavy pipe spans is difficult for laying offshore pipelines in very deep waters.
The diameter-to-thickness ratio of a pipe determines whether it will have a negative or positive buoyancy. In depths of up to about 500 feet, it is common for a pipe to have a diameter-to-thickness ratio in the range from 40 to 100, which means that the pipe would float. Such a pipe requires a thick concrete coating to give it negative buoyancy. A diameter-to-thickness ratio of about 29.5 is the break-even point, i.e. below this ratio, the pipe will sink and above this ratio, the pipe will float. At about 2,000 feet, a diameter-to-thickness ratio of about 26 is needed just to resist external pressure, and of course, at this depth no concrete is required. At 3,000 to 4,000 feet, a diameter-to-thickness ratio of about 20 is needed to resist external pressure, and the pipe used at these depths is thus quite heavy.
Prior art pertinent to the present invention includes U.S. Pat. Nos. 2,910,835; 3,262,275; 3,454,051; 3,530,680; 3,550,639; 3,599,435; 3,847,185; 4,009,736.